The invention relates generally to the field of liquid filtering systems, and, more particularly to filtering systems that allow for on-line switching between discrete filters such that overall process flow is uninterrupted.
In the manufacture of photographic products, such as photographic films and papers, support webs are coated with one or more layers in a continuous operation. A variety of coating apparatus may be used including, for example, curtain-coating apparatus, bead coating apparatus, and extrusion coating apparatus. All of these coating methods require a constant source of clean coating solution. Traditionally, after the coating solution or composition is prepared, it is filtered as it is delivered to the coating process. If a single filter is used, the filter path becomes blocked as a result of the filtering action. In other words, as the filter collects contaminating particulates from the coating solution, the greater the back pressure caused by the filtration operation. At some point, the filter becomes so blocked that flow of coating solution to the coating machine must be stopped to allow the filter to be removed and either changed or cleaned. Therefore, it is typical for such coating operations to now employ two or more filters arranged in parallel, thereby allowing switching from a first (plugged) filter to a second (clean) filter on-line without interruption of coating solution flow through the process. In this filter switching operation, it is typical that the second filter be prepared by flowing a coating solution there through to drain while the first (plugged) filter continues to filter solution supplied to the coating machine. Without preparation of the second filter, air bubbles would be delivered to the coating machine resulting in severe coating defects in the coating product. When the second filter is fully prepared, as determined through the detection of coating solution at the drain, the second filter is brought on-line and flow through the first (plugged) filter is terminated. The plugged filter is then replaced. In that manner, the first filter stands ready to filter coating solution when the second filter becomes plugged.
Although the prior art filtering method discussed above allows for uninterrupted flow of coating solution to the coating machine, switching from one filter to another arranged in parallel has the drawback of requiring that coating solution be flowed to drain during filter preparation and thereby wasted.
It is therefore an object of the present invention to provide a method for switching between filters arranged in parallel that substantially eliminates waste of coating solution.
It is a further object of the present invention to provide a method for switching between filters in a coating operation that eliminates waste of coating solution from filter preparation while still preventing introduction of bubbles to the coating machine.
Briefly stated, these and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by employing a multiport filter selection valve upstream of the parallel filters in the process, a multiport filter diverter valve downstream of the parallel filters in the process, and a flow surge device downstream of the filter diverter valve but upstream of the coating machine. With this hardware, when it is determined the first filter will soon become inoperable, a concurrent flow path through the filter selection valve is opened such that coating solution is simultaneously flowed through both the first and second filters. A concurrent flow path through the filter diverter valve is opened such that while coating solution flowing through the first filter is still being supplied to the coating machine, the flow path for coating solution flowing through the second filter is to drain. Once the flow path to drain through the second filter is opened, however, the port open to drain in the filter diverter valve remains open for a fixed period of time. That fixed period may be predetermined by calculation, knowing the volume of the second filter and the associated conduit path through the filter selection valve and the filter diverter valve as well as the flow rate of the solution therethrough. In this manner, the process computer or programmable logic controller (PLC) controlling actuation of the filter selection valve and the filter diverter valve can time the closing of the port open to drain in the filter diverter valve such that substantially all of the air in such flow path has been driven therefrom without actually sending any coating solution to drain. The port in the filter diverter valve to the downstream process is simultaneously opened. At this point, coating solution is flowing through both filters to the surge device downstream thereof. Then the appropriate ports in the filter selection valve and the filter diverter valve are closed such that the process flow path is no longer though the first filter and all flow of coating solution to the surge device is through the second filter. With the first filter now out of service, an operator can replace the spent filter with a new or cleaned filter thereby readying it for a reverse switching operation when the second filter becomes plugged.
The determination of the period of time that the flow path through a new filter is open to drain will vary depending not only on the flow rate and the volume of the filter, but the type of filter media as well. The process computer or PLC may be preprogrammed with the critical information for a variety of filters, filter sizes and filter media. The flow rate may be predetermined and input into the process computer or PLC, or alternatively, the flow rate may be measured with well-known flow measurement devices transmitting such information to the process computer or PLC.